As substrates used in the production process of semiconductor devices, wafers made of a semiconductor such as silicon wafers are widely used. As for such a wafer, polished wafers (PWs) obtained by slicing a single crystal ingot and mirror polishing the slices, epitaxial wafers obtained by forming an epitaxial layer on a surface of a PW, and the like are known. For example, epitaxial wafers are used as device substrates of various semiconductor devices such as metal oxide semiconductor field-effect transistors (MOSFETs), dynamic random access memories (DRAMs), power transistors, and back-illuminated solid-state imaging devices. Note that while “epitaxial wafer surface”, “front surface”, or simply “surface” of an epitaxial wafer herein refers to one side of the main surfaces of an epitaxial wafer, where an epitaxial layer is formed. Whereas, “epitaxial wafer back surface” or “back surface” of the epitaxial wafer refers to the other side of the main surfaces of the epitaxial wafer, opposite to the surface where the epitaxial layer is formed (i.e., surface where the epitaxial layer is not formed).
In terms of enhancing yield and reliability of semiconductor device manufacturing processes, inspection techniques for detecting defects in the front and back surfaces of wafers used as substrates for semiconductor devices have increasingly become very important. Various defects are formed in the front and back surfaces of a wafer. Examples include crystal defects such as pits and COPs, unevenly polished portions and scratches formed due to machining, and adhesion of particles that constitute foreign matter.
Conventionally, using a light point defect (LPD) inspection apparatus (laser surface inspection system), wafer inspection is performed in which wafer surfaces having been finished by mirror polishing are scanned with laser light thereby detecting scattered light resulted from particles, scratches, and the like in the front and back surfaces. Further, in order to determine the presence and absence of defects that are hardly determined by an LPD inspection apparatus, appearance inspection is also performed in which the wafer surfaces are examined by visual observation. Since appearance inspection is an organoleptic test, variation in the determination depending on inspectors cannot be avoided, and it takes time for inspectors to master the examination technique. Therefore, there is a demand for establishing an objective inspection method and an automatic inspection method.
To address the above, we have previously proposed in JP 2010-103275 A (PTL 1), as a wafer inspection method, a method of properly evaluating wafers without appearance inspection especially focusing on defects on the rear surface side of the wafer surfaces. Specifically, the method is a method of evaluating the rear surface of a wafer, including: a mapping step of consecutively taking partial images of the rear surface of a wafer in the circumference direction of the wafer and composing the taken partial images to compose a full image of the rear surface of the wafer; and a differentiation step of differentiating the full image to create a differentiated image of the rear surface of the wafer, wherein the wafer is evaluated by detecting unevenly polished portions, haze, scratches, and particles based on the full image or the differentiated image.
An optical system 50 for creating the above full image is described with reference to FIGS. 1A and 1B. FIG. 1B depicts the major part of FIG. 1A for illustrating irradiation light L1 emitted by an annular fiber optic illuminator 51 and reflected light (scattered light) L2. This optical system 50 includes an annular fiber optic illuminator 51, a lens barrel 52, a telecentric lens 53, and a light receiving unit 54. An extra high pressure mercury lamp (wavelength range: 369 nm to 692 nm, output: over 1,000,000 lux) is used for a light source of the annular fiber optic illuminator 51, and a CCD camera is used for the light receiving unit 54. The irradiation light L1 emitted by the annular fiber optic illuminator 51 enters a wafer W at an angle of approximately 20° to the wafer plane and turns into the scattered light L2 upon colliding with a defect D present in the rear surface of the wafer W. The first light receiving unit 54 takes an image upon receiving perpendicular scattered light of the scattered light L2, thereby obtaining and storing the image containing the information of the position of the first optical system 50 and the brightness information of the scattered light.